C-172 landing light switch

My '61 Cessna landing light switch has two positions; First notch is the taxi
light, and the last notch turns on the landing light but leaves on the taxi
light. Does anyone know of a legal replacement switch that separates the two
lights? (taxi light goes off on the last notch). The reason I ask is that it
still has the generator and at engine idle the generator circuit breaker often
pops due to the high load and low output. A separating switch would presumably
cut the load way down.

Jim

JFLEISC wrote:
My '61 Cessna landing light switch has two positions; First notch is the taxi
light, and the last notch turns on the landing light but leaves on the taxi
light. Does anyone know of a legal replacement switch that separates the two
lights? (taxi light goes off on the last notch). The reason I ask is that it
still has the generator and at engine idle the generator circuit breaker often
pops due to the high load and low output. A separating switch would presumably
cut the load way down.

Ummm. I don't know, but isn't the circuit breaker sensitive to the amount of
current from the generator? If there is "low output" how could the circuit
breaker be opening?

Assuming the generator and circuit-breaker were originally sized to handle the
load of both lights on simultaneously, my guess is that the circuit breaker is
worn out and tripping when it shouldn't, or the lights are imposing a larger
load than designed for (wrong bulb?).

Dave
Remove SHIRT to reply directly.

my guess is that the circuit breaker is
worn out and tripping when it shouldn't,

The circuit breakers are new (originally a '61 C-172 has fuses). Remember that
it is the generator breaker, not the bulb breaker that is tripping. Also, it
never trips when the idle speed is kept up (of course this wears out the
brakes).

Jim

JFLEISC wrote:
My '61 Cessna landing light switch has two positions; First notch is the taxi
light, and the last notch turns on the landing light but leaves on the taxi
light. Does anyone know of a legal replacement switch that separates the two
lights? (taxi light goes off on the last notch). The reason I ask is that it
still has the generator and at engine idle the generator circuit breaker often
pops due to the high load and low output. A separating switch would presumably
cut the load way down.

Jim

Jim,

first, a std 4509 landing light draws about 8A. Check to see if some
has installed Q4509 or some other non-standard sealed beam.

second, all things working normally, if the total load exceeds the
total generation capability, then the difference is supposed to come
from the battery (as long as it has some residual charge in it). In
other words, if the total bus load is 40A, but your generator is capable
of delivering only 30A, then the genny should be up to delivering its
full rated 30A output all day long. It shouldn't care what the total bus
load is... (turning on extra load should not trip the genny breaker)

If the genny blows its breaker, then either the breaker is bad (more
on this later), or someone has dicked with the current cutout relay
in the generator regulator in a misguided attempt to get it to
deliver more output. The current cutout relay must be set lower than
the Gen breaker rating.

As breakers age, they can develop internal resistance in their contacts.
The heat produced by the increased contact resistance adds directly to
the heat produced by the little built in heater (that is why they are
called a thermal breaker), and the breaker will trip at currents lower
than what is engraved on the button...

You need to find an A&P who understands electricity (not many of them do).

MikeM, PhD EE
Skylane '1MM
Pacer '00Z

but your generator is capable
of delivering only 30A, then the genny should be up to delivering its
full rated 30A output all day long. It shouldn't care what the total bus
load is... (turning on extra load should not trip the genny breaker)

No argument here. But lets say that the FULL rating is 30A. What is the output
if it is spinning at %10 of that full rated speed? How much current will now be
going over those oxidized breaker contacts that seemed to handle the full speed
output current OK?

Jim

JFLEISC wrote:

No argument here. But lets say that the FULL rating is 30A. What is the output
if it is spinning at %10 of that full rated speed? How much current will now be
going over those oxidized breaker contacts that seemed to handle the full speed
output current OK?


The max available gen output current should be (roughly) proportional to
RPM, meaning that at low RPM, it should produce way less than
its full rated current. No way should it trip off line just because
its turning slow.

The old genny regulators have at least two relays; one to limit the
max output voltage, and the other to limit the max output current.
I'm guessing that your current cutout relay is either welded closed,
or someone has dicked with it, and it never opens.
However, the Voltage cutout relay is working correctly at 14V.

How's this for a scenario: at high RPM, the bus (battery) voltage is
near 14V, so the voltage cutout senses the bus voltage (correctly),
causing the generator to cycle on/off, thereby limiting the average
current to less than it takes to trip your Gen breaker.

As you slow the engine, the battery voltage sags below the level which
causes the voltage sense to cutoff, leaving the generator to deliver
whatever current it can at that rpm, which may still be in excess of
what the Gen breaker can carry. If the current cutout relay was working
properly, it would sense the excessive current, and then cycle the
gen output on/off so as to reduce the average current through the
Gen breaker.

MikeM

"mikem" wrote in message
...

first, a std 4509 landing light draws about 8A. Check to see if some
has installed Q4509 or some other non-standard sealed beam.

Good...

second, all things working normally, if the total load exceeds the
total generation capability, then the difference is supposed to come
from the battery (as long as it has some residual charge in it). In
other words, if the total bus load is 40A, but your generator is capable
of delivering only 30A, then the genny should be up to delivering its
full rated 30A output all day long. It shouldn't care what the total bus
load is... (turning on extra load should not trip the genny breaker)

But after that extra load (coupled with low engine RPM), has dragged down
the battery voltage, all bets are off. The regulator might then put more
currrent to the field (this current may be passing through that breaker),
causing the alternator current to drasticly increase.


If the genny blows its breaker, then either the breaker is bad (more
on this later), or someone has dicked with the current cutout relay
in the generator regulator in a misguided attempt to get it to
deliver more output. The current cutout relay must be set lower than
the Gen breaker rating.

As breakers age, they can develop internal resistance in their contacts.
The heat produced by the increased contact resistance adds directly to
the heat produced by the little built in heater (that is why they are
called a thermal breaker), and the breaker will trip at currents lower
than what is engraved on the button...

Another possibility is a small amount of resistance where the wires connect
to the breaker, this will cause heat (Power = I ^2 X R) which will have the same
effect as bad contacts in the breaker. You find this by feeling for excessive
heat when there is current flowing through the breaker. The breaker may
actually make the panel noticably hot, or you may have to feel behind the panel
(taking all necessary safety precautions).

Vaughn (Licensed Electrician, Electronics Tech, but NOT an A&P)




You need to find an A&P who understands electricity (not many of them do).

MikeM, PhD EE
Skylane '1MM
Pacer '00Z

At low RPM the cut out relay in the regulator is SUPPOSED to open so
that the battery does not BACKFEED into the generator. This is relay
is apparently not opening. This relay has two windings, one of many
turns of fine wire and the other is a few turns of heavy wire. When
the generator is charging the two windings add to keep the relay
closed. When the generator RPM is low the battery back feeds the
generator and tries to run it as a motor. When this happens the
fields of the two windings oppose each other and the armature contact
to the battery opens. Since the circuit breaker has opened so many
times it most likely will not hold rated current either.

REPLACE THE VOLTAGE REGULATOR AND THE CIRCUIT BREAKER.

JF

On 03 May 2004 23:04:47 GMT, (JFLEISC) wrote:

but your generator is capable
of delivering only 30A, then the genny should be up to delivering its
full rated 30A output all day long. It shouldn't care what the total bus
load is... (turning on extra load should not trip the genny breaker)

No argument here. But lets say that the FULL rating is 30A. What is the output
if it is spinning at %10 of that full rated speed? How much current will now be
going over those oxidized breaker contacts that seemed to handle the full speed
output current OK?

Jim

The old genny regulators have at least two relays; one to limit the
max output voltage, and the other to limit the max output current.
I'm guessing that your current cutout relay is either welded closed,
or someone has dicked with it, and it never opens.
However, the Voltage cutout relay is working correctly at 14V.

How's this for a scenario: at high RPM, the bus (battery) voltage is
near 14V, so the voltage cutout senses the bus voltage (correctly),
causing the generator to cycle on/off, thereby limiting the average
current to less than it takes to trip your Gen breaker.

As you slow the engine, the battery voltage sags below the level which
causes the voltage sense to cutoff, leaving the generator to deliver
whatever current it can at that rpm, which may still be in excess of
what the Gen breaker can carry. If the current cutout relay was working
properly, it would sense the excessive current,

This is starting to make sense.
The last question is (and it makes sense that it should still work this way)
what part of the circuit does the main power shutoff switch kill? Does it cut
out the current circuit as well as the aircraft power supply solenoid? It seems
to me that if the current ("antibackfeed") contacts were welded closed then
the circuit breaker would pop when the main power switch was pulled on even
with the engine off (it doesn't). Any thoughts here? (I'm not the type of
person who 'throws' parts at a problem until its fixed, unless someone else is
paying:-))

The cutout relay opens the path between the armature and the battery.
This relay closes when the generator RPM is high enough to generate a
voltage that is equal or exceeds the battery voltage. When the cutout
relay closes the heavy wire coil on the relay adds more flux to keep
the relay closed. When the generator RPM decreases so that the
generator voltage is less than the battery voltage, the battery tries
to "motor" the generator and the current through the heavy wire
winding reverses and opposes the flux from the fine wire winding
causing the relay to open which disconnects the generator from the
battery.
John

On 04 May 2004 01:19:50 GMT, (JFLEISC) wrote:

The old genny regulators have at least two relays; one to limit the
max output voltage, and the other to limit the max output current.
I'm guessing that your current cutout relay is either welded closed,
or someone has dicked with it, and it never opens.
However, the Voltage cutout relay is working correctly at 14V.

How's this for a scenario: at high RPM, the bus (battery) voltage is
near 14V, so the voltage cutout senses the bus voltage (correctly),
causing the generator to cycle on/off, thereby limiting the average
current to less than it takes to trip your Gen breaker.

As you slow the engine, the battery voltage sags below the level which
causes the voltage sense to cutoff, leaving the generator to deliver
whatever current it can at that rpm, which may still be in excess of
what the Gen breaker can carry. If the current cutout relay was working
properly, it would sense the excessive current,

This is starting to make sense.
The last question is (and it makes sense that it should still work this way)
what part of the circuit does the main power shutoff switch kill? Does it cut
out the current circuit as well as the aircraft power supply solenoid? It seems
to me that if the current ("antibackfeed") contacts were welded closed then
the circuit breaker would pop when the main power switch was pulled on even
with the engine off (it doesn't). Any thoughts here? (I'm not the type of
person who 'throws' parts at a problem until its fixed, unless someone else is
paying:-))